Those who are terminally ill or undergoing chemotherapy or radiotherapy are being threatened with benefit cuts if they do not attend the meetings, according to Macmillan Cancer Support and Citizens Advice.

The "fit for work" interviews are for people seeking the employment and support allowance (ESA), which replaced incapacity benefit and income support in October 2008.

The drive behind ESA is to focus on what people can do rather than what they cannot do, as a means of getting them back to work.

However, cancer sufferers undergoing chemotherapy or radiotherapy or who are terminally ill are automatically exempt from the interviews.

Macmillan and Citizens Advice condemned the ESA process, saying it was "failing seriously ill and disabled people". Macmillan's benefits helpline has taken more than 600 calls about the issue since May.

A joint report - Failed by the System - found evidence of cancer patients with just months to live being told they had to undergo medical examinations and be questioned. Others having radiotherapy and people in hospital have also been refused ESA when they should automatically get it, the study found.

It also noted examples of people with cancer being told they are fit for work even when they are suffering from the long-term effects of the disease.

The charities said poor knowledge of ESA rules among Jobcentre Plus and Department for Work and Pensions medical staff is resulting in claims being handled badly. Poor administration systems and a lack of understanding about cancer are fuelling the problem, they said.

Mike Hobday, head of campaigns at Macmillan, said: "It's cruel and completely unacceptable that people who are terminally ill or going through gruelling treatment are being made to jump through hoops to get money they should receive automatically. The safeguards to protect cancer patients clearly aren't working, and the ESA system is riddled with problems."